Chinese acrobats, Italian judges, and traffic jams.

You might want to reconsider saying yes to the latest project that your boss drops on your desk like a side of beef. Saying no might help you do a better -- or at least a faster -- job.
Turns out that managing so many concurrent projects that you're the white-collar equivalent of a Chinese acrobat spinning dishes doesn't work so well.

A study of Italian judges who were randomly assigned cases and who had similar workloads found that those who worked on fewer cases at a time tended to complete more cases per quarter and took less time, on average, to complete a case. The authors concluded that

Individual speed of job completion cannot be explained only in terms of effort, ability and experience: work scheduling is a crucial “input” that cannot be omitted from the production function of individual workers.

excessive multitasking may result in the workflow equivalent of a traffic jam, where projects get backed up behind other projects much the way cars get stuck in traffic when there are too many on a highway at once.

If this phrasing rings a bell, it should: here's how Jim and Tonianne made this point visually (check out slide #7):

A few weeks ago, I wrote about the need to use your calendar as a tool to assess your daily production capacity, but not with the goal of filling up every minute of each day. Overloading the system writ small -- stacking up tasks during the day like 747s over LaGuardia -- is a bad idea. But overloading the system writ large -- scheduling too many legal cases or too many projects at one time -- is also a recipe for slow turnaround, frustrated customers, sub-optimal performance, and probably premature hair loss.

Remember, you're not a circus performer. Neither your boss nor your customers "ooh" and "ahh" because you're juggling 26 projects at once. They ooh and ahh when you deliver the goods quickly and with perfect quality.